At present, in mobile communication, a number of new techniques and applications such as OFDM and MIMO are developed with the development of theory and technique. These techniques significantly improve the performance of mobile communication systems and respond to requirements for radio multimedia and high-speed data transmission. Multiple-Input Multiple-Output (MIMO) technique is a significant achievement in intelligent antenna technique in a radio mobile communication field. The MIMO technique indicates the use of a plurality of antennas for both data transmission and data reception. As a result of studies, by using the MIMO technique, it is possible to increase the channel capacity, improve channel reliability and reduce a bit error rate. The maximum capacity or upper limit of the capacity of a MIMO system linearly increases in accordance with the increase in the minimum number of antennas. Under the same condition, when a normal intelligent antenna such as a multi-antenna and antenna array is used on a receiving side or transmitting side, the capacity increases according to the increase in the logarithm of the number of antennas. On the other hand, the MIMO technique has an extremely high potential for an increase in the capacity of a radio communication system and is thereby a key technology used in new generation mobile communication.
Data traffic requirements resulting from the development of communication have increased day by day and methods for supporting high-speed data traffic have been also proposed one after another. In the methods, higher order modulation techniques such as 8PSK and 16QAM are commonly used. Although the higher order modulation techniques improve a system data transmission rate, the techniques require a high signal-to-noise ratio. Hence, the use of the higher order modulation techniques is limited. In the higher order modulation techniques, n encoded bits are mapped onto a certain constellation diagram to produce higher order modulation symbols. Further, the Euclidean distance between adjacent points in a constellation diagram becomes smaller in accordance with an increase in M-ary number of higher order modulation. Therefore, in the higher order modulation techniques, under the condition that the signal-to-noise ratio is the same, the symbol error rate becomes higher for higher order modulation. The symbol error rate for higher order modulation has a relationship with a minimum Euclidean distance in a constellation diagram. On the other hand, the symbol error rate for higher order modulation has no relationship with a mapping relationship between encoded bits in a constellation diagram. However, the encoded bit error rate of a higher order modulation symbol has a strong correlation with the mapping relationship between encoded bits in a constellation diagram. Accordingly, for the encoded bit error rate in a higher order modulation symbol, different encoded bit error rates corresponding to n encoded bits can be obtained by performing mapping differently.
Data traffic requirements for a transmission error rate are very high. For example, a frame error rate is required to be 0.1 percent. Therefore, it is necessary to use channel coding and error correction techniques for achieving such high performance in a poor radio channel environment. At present, a hybrid automatic repeat request (HARQ) technique is generally used. The technique is for detecting and correcting errors by combining an automatic repeat request (ARQ) technique and a forward error correction (FEC) technique. Currently, there are the following three types of hybrid automatic repeat request technique. In the first type, a receiving side discards a packet that cannot be received properly, informs a transmitting side through a return channel to retransmit a copy of the original packet, and independently decodes a newly received packet. In the second type, a receiving side does not discard an error packet and decodes the packet in combination with retransmitted information. In the third type, a receiving side does not discard an error packet and decodes the packet in combination with retransmitted information. In this case, a retransmitted packet includes all essential information to receive data properly.
When a channel error correction is performed using HARQ, first, a transmitting side transmits encoded information to a receiving side. When the receiving side receives the information, the receiving side performs error correction decoding on the received information. If the receiving side properly receives data, the receiving side receives the information and transmits ACK acknowledgement information to the transmitting side. On the other hand, if the receiving side cannot correct an error, the receiving side requests the transmitting side to retransmit data by transmitting NACK information to the transmitting side. Then, the receiving side performs decoding again based on received retransmission data.
FIG. 1 illustrates the configuration of a MIMO+HARQ system of the prior art.
In the above-described configuration, on a transmitting side, transmission data passes through serial/parallel conversion section 101 and is divided into nT data substreams. Each data substream has a one-to-one correspondence with a transmitting antenna. Before transmission, CRC coding section 102 performs CRC coding on the data substreams. Encoding/modulation section 103 performs encoding and modulation. Then, the modulated data is transmitted through nT antennas 104. Information fed back by feedback channel 111 indicates a reception state of data. The transmitting side determines whether or not to retransmit the data based on the reception state.
A receiving side receives all signals in a space through nR receiving antennas 105. Channel estimation section 106 performs channel estimation based on pilot signals included in the received signals or using other methods. Further, channel estimation section 106 estimates current channel characteristic matrix H. In a MIMO system, channel characteristics can be described using a matrix. Finally, MIMO detection section 107 performs detection on each transmission data substream based on channel characteristic matrix H. CRC check section 108 performs a CRC check on the detected data. Feedback information processing section 110 generates feedback information using information depending on whether or not decoding can be performed properly. The receiving side transmits the feedback information to the transmitting side through feedback channel 111. Properly received data passes through parallel/serial conversion section 109 and is ultimately restored to the original data.